Narrowing an 8 1/4" rear end, how is it done?

Being a 67 Barracuda and not a Dart, you can use a early "B" like a 1963 Housing.and just install new perches.
It's one of the narrow ones 62-65. Check with Dr.Diff web site for all the information and axle kits to do the job.
You can leave him (Cass) a e-mail. He usually responds in a day or so.Great guy.